The present invention relates to a control system for an eddy current coupling drive for armoured face conveyors.
It is known to use eddy current coupling drives for conveyors and such couplings provide a controlled build up of torque and/or speed, and enable the load to be shared reasonably equally between drives when more than one drive is used on the conveyor. When applying the eddy current coupling drive to armoured face conveyors, account must be taken of the fact that the weight of excavated material on the conveyor can increase the frictional resistance and inertia such that considerable excess torque may be required to start the conveyor. It may not be possible to control, consistently, the required torque with the available eddy current coupling drive control system and in this situation the motors may stall. The supply voltages to the motors of several coupling drives may not be identical, resulting in varying maximum available torques, and thus making the application of maximum total torque more difficult. In addition, in longwall mining the drives may be at each end of the armoured conveyor, and these can be separated by several hundred metres. Normally, each eddy current coupling is supplied from a separate control unit which incorporates some form of control of a thyristor rectifier which supplies d.c. current to the excitation coils of the coupling. In a multi-motor drive as is used in armoured face conveyors, these control units are interlinked electrically, and may be mounted in a common enclosure with supply cables being connected to each coupling. If it is also desired to monitor speed signals from the coupling drive, this normally requires another cable between the control units and each coupling drive.
Similarly, measurement of the motor terminal voltage as a control parameter requires further cable between the coupling drives and the control unit. This accumulation of cables between the control units and the drives is impractical in the use of an armoured face conveyor.
It is an object of the present invention to obviate or mitigate the limitations of the existing conveyor control system.